Orthodontic appliances represent a principal component of corrective orthodontic treatment devoted to improving a patient's occlusion. One type of orthodontic appliance is an orthodontic bracket. Using the orthodontic bracket as an example, an orthodontist may affix orthodontic brackets to the patient's teeth and engage an archwire (another type of orthodontic appliance) into a slot of each bracket. The archwire applies corrective forces that coerce the teeth to move into orthodontically correct positions. Traditional ligatures are often employed to retain the archwire within each bracket slot. Due to difficulties encountered in applying an individual ligature to each bracket, self-ligating orthodontic brackets have been developed that eliminate the need for ligatures by relying on a movable latch or slide for captivating the archwire within the bracket slot. Other orthodontic appliances include palatal expanders, Temporary Anchoring Device (TAD) attachments, bands, torquing springs, and Herbst appliances, among others.
The design, and ultimately the performance, of many of these orthodontic appliances are often limited by the process by which the appliance is manufactured. Generally, manufacturability of an orthodontic appliance is gauged in terms of large quantities (e.g., tens of thousands of units) and to a lesser extent on prototype or testing quantities. But in any respect, the appliance must be capable of being made before it may leave the conceptual stage of development. So, while there may be many theoretical orthodontic appliance designs that may theoretically improve orthodontic treatment, if such a design cannot be brought from within the conceptual world into the real world, and done so at a reasonable cost, it may be abandoned during product development.
Orthodontic appliances are often made in large numbers by manufacturing processes that include metal injection molding (MIM), ceramic injection molding (CIM), casting, and laser sintering (e.g., direct metal laser sintering). However, each of these processes is limiting in a way that restricts the manufacturable appliance design. Thus, many desirable features are not included in the design actually manufactured. Still other processes include machining, stamping, and welding. These other processes also limit the manufacturable orthodontic appliance design.
In particular, in MIM or CIM, metal or ceramic particles, respectively, are mixed with a binder to form a slurry. The slurry is then injected into a mold having the shape of an orthodontic appliance or a component thereof. Similarly, in casting, a molten material (e.g., molten metal) is forced into a mold. Thus, forming an orthodontic appliance or associated component made by any of these processes requires a mold. Use of a mold limits the features of the appliance to those that are formable by and also separable from the mold. Generally, this limits the type and orientation of both internal and external features of the appliance.
Where the feature of the appliance or component is formed by a mold, as opposed to a feature machined into the appliance, the corresponding mold feature must be initially created in the mold surface. There is a lower size limit and minimum level of detail capable of being formed in the mold depending on the method by which the mold is made. Furthermore, the design of the component itself is also restricted by the mold design. For the component to separate from the mold during a demolding process, the component must have a relief or converging aspect that allows the component to be removed from the mold cavity. Otherwise, that is, where there is an interference fit or diverging geometry between the component and the mold, the component would not be removable from the mold cavity. In essence, the component and the mold would be locked together, necessitating destruction of the mold or a portion thereof to release the component. Thus, the use of a mold limits the kind and orientation of features capable of being manufactured.
Additional limitations are associated with use of a mold. For example, the mold design must also incorporate a location for the material to be injected into the mold cavity (e.g., through a gate or other opening) and must also account for how the component, once formed, is removed or ejected from the mold (e.g., via an ejection pin). Any of the above features of the mold may create defects in the component (e.g. flash) that must be removed in a subsequent process. These additional design considerations significantly restrict the features of the component that are ultimately manufactured.
However, alternative geometries that are not capable of being formed during molding may be desirable. The desired geometries may enhance the use and functionality of the orthodontic appliance. For example, certain undercut formations, which include various voids or cavities, may not be capable of being formed in a mold. In some instances, these features may be formed using post-formation processes, such as machining. However, such post-formation processes are generally time consuming and expensive. Still other desirable features are not available at any cost because of the limits imposed by the above-mentioned manufacturing techniques. These desirable features are thus ultimately excluded from a production or real-world design.
In addition, MIM, CIM, and casting processes restrict the materials used in forming the component. Each component manufactured by one of these processes is made by a single shot of a material (e.g. metal particles or ceramic particles or molten material, respectively). The component is consequently made of a single material, though in limited circumstances various inserts may be encapsulated during injection of the slurry or molten material. By contrast, a single component made of multiple materials during a single shot or injection is generally not possible.
A related limitation associated with molding is when the orthodontic appliance is made of multiple components. A multi-component appliance requires assembly following the separate molding of the individual components. Typically this is the situation where various components of the appliance move relative to one another, for example, a self-ligating orthodontic bracket. The molding processes set forth above are not capable of forming the components in their relative assembled configurations. Furthermore, due to the relatively small sizes of the components (often these components are millimeters or fractions of millimeters in size), post-manufacturing assembly can be problematic and may add to the expense of manufacturing the orthodontic appliance.
In addition, in each of the injection molding processes, the component dimensions are difficult to control over both the short term as well as the long term. For example, the green body resulting from a MIM or CIM process is oversized compared to the final product. The green body is sintered to densify the metal or ceramic particles in the slurry. During sintering, the green body shrinks as it approaches a usable or theoretical density. This shrinkage can be affected by any one of a number of variables. Lack of sufficient control of any single variable can cause problems with the dimensions of the component. Thus, producing predictable, consistent shrinkage during production of the component over both the short and long term is difficult. And, due to the number of variables, optimization of the slurry and the mold size requires extensive up-front experimentation. Thus, it may take months before a process for making a new appliance is ready for mass production. Further, once the initial process is established, which provides the desired dimensions, mold wear contributes to reduction in the precision of the desired dimensions over the long term. Accordingly, ongoing maintenance of the molds is necessary to combat mold wear and thus adds to the manufacturing costs.
Other processes used to manufacture orthodontic appliances include rapid prototyping processes, such as, direct metal laser sintering. This process produces components directly from the metal and is essentially a free-form process—no mold is required. In this aspect, many of the problems associated with molds are eliminated. However, rapid prototyping processes still have shortcomings.
Rapid prototyping processes build products on a layer-by-layer basis. The layers are produced in a serial fashion. That is, the layer is produced by tracing a laser over the layer and requires the particles of material to be near the point of intersection between the laser and the layer being formed. The focus point of the laser melts or sinters the particles as the laser traverses along a predetermined path. However, the full layer is not produced until the laser has traversed the entire path for that layer. At any given moment, prior to complete traversal, the layer is only partially formed.
However, rapid prototyping processes introduce unique problems in the manufacture of products made thereby. For one, location of the particles or accuracy of the powder injection with respect to the laser creates dimensional issues. Also problematic is the poor finish quality of the as-formed surface. The poor finish necessitates a subsequent finishing operation. Again, this subsequent operation adds cost to the manufacturing process. In addition, significant thermal gradients develop in the part, often causing distortion in the component or unintended changes in the material phases present in the part.
There is a need, therefore, for a method for manufacturing an orthodontic appliance that addresses these and other problems associated with conventional manufacturing methods. Furthermore, there is a need for a manufacturing method that does not limit orthodontic appliance design. More particularly, there is a need for orthodontic appliances having features that are not capable of being formed in single shot injection molding processes or by rapid prototyping process such that as-yet unmanufacturable design features can be manufactured and then used in the clinical setting.